 A vibration of the Earth produced by a rapid release of energy  Often occur along faults – breaks in the Earths crust and mantle (plate boundaries)

 1. Focus – Point within the Earth where the earthquake starts ◦ Energy moves in all directions from this point  2. Epicenter – Point on the Earth’s surface directly above the focus ◦ Most intense movement during the earthquake

 The Elastic Rebound Hypothesis ◦ Stress builds along an existing fault ◦ Deformation of crustal rock – bent features of the rocks caused by increasing stress – elastic energy ◦ Slippage (earthquake) – energy is stored in the rock overcomes frictional forces keeping crust in place ◦ Energy released – rock returns to original shape

 At spreading ridges, or similar extensional boundaries, earthquakes are shallow, and tend to be smaller than magnitude 8.

 At compressional boundaries, earthquakes are found from the very near surface to several hundred kilometers depth, ◦ since the coldness of the subducting plate permits brittle failure down to as much as 700 km.  Compressional boundaries host Earth's largest quakes, with some events on subduction zones in Alaska and Chile having exceeded magnitude 9.

 At transforms, earthquakes are shallow, as deep as 25 km; mechanisms indicate strike-slip motion. ◦ Tend to have earthquakes smaller than magnitude 8.5.  The San Andreas fault in California is an example of a transform, separating the Pacific from the North American plate.  At transforms the plates mostly slide past each other laterally, producing less sinking or lifting of the ground than extensional or compressional environments. ◦ The yellow dots below locate earthquakes along strands of this fault system in the San Francisco Bay area.

 Measured by a seismograph (instrument)  Seismographs produce seismograms – the written record of the movement

 Surface Waves – earthquake waves that travel along the Earth’s outer layer ◦ Up and down, side to side, twisting motion ◦ Most destructive of the waves ◦ Slowest wave

P waves: (Body waves)  Compression waves  Alternately expand and compress material they pass through  Travel the fastest of all three waves  Travels parallel to its movement

S Waves (Body waves) Transverse wave Travel slower than P waves Travels at a right angle to the direction of its movement Will not travel through liquids and gases

 Duration  Intensity  Building Design – reinforced/flexible buildings best  Materials built on – hard, dense material the best  Liquefaction - when loose sediments are saturated with water, during an earthquake, shaking creates a liquid like material not able to support structure – structure sinks

 Richter scale: This scale was based on the ground motion measured ◦ there is an upper limit on the highest measurable magnitude, and all large earthquakes will tend to have a local magnitude of around 7. ◦ the magnitude becomes unreliable at a distance of more than about 600 km (370 mi) from the epicenter  Moment Magnitude Scale (M W or M) is used to measure the size of earthqukes in terms of the energy released ◦ Rigidity of Earth x Amount of Slip x Area that slipped

 caused by an earthquake that displaces the ocean floor vertically ◦ Convergent and strike-slip boundaries  water is pushed upwards and toward the land  speeds of 500/9000 km/hr  low waves in open water – as wave nears shore water builds upwards

 Fires - from broken gas pipes and falling power lines  Landslides – rock and soil slide downhill from shaking

 No predicting devices  Only measurements – stress along faults, water level in wells, gas emissions from fractures  Seismic gap – time between earthquakes  Foreshocks-small quakes preceeding big one ◦ Could be hours, days, even years before  Aftershocks-small quakes following big one

 Knowledge of Earth’s interior comes from Earthquake waves  By measuring the speed of a wave - able to determine the composition of the Earth  P waves travel through liquids and solids  S waves travel only through solids

 As P waves travel they bend as they enter new material - bending shows change of material  S waves not traveling through the outer core also shows change in material  Conclusion – Earth is made up of different states of material